Electronically controllable limited slip differential having nonmagnetic casing portion

ABSTRACT

A controllable differential having a rotatable casing, differential gearing disposed within the rotatable casing and linking the casing with an output element disposed therein, a clutch disposed within the rotatable casing and clutch adapted to transfer torque between the rotatable casing and the output element, and an electromagnet arranged to generate a generally toroidal magnetic flux path encircling the electromagnet and magnetically forcing the clutch into engagement with the rotatable casing, the casing including a nonmagnetic portion adjacent the electromagnet, said flux path surrounding the nonmagnetic casing portion.

This is a division of application Ser. No. 09/030,602, filed Feb. 25.1998.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to differentials, and more particularly,to traction enhancing differentials.

2. Description of the Related Art

Differentials are well known in the prior art and allow a pair of outputshafts operatively coupled to an input shaft to rotate at differentspeeds, thereby allowing the wheel associated with each output shaft tomaintain traction with the road while the vehicle is turning. Such adevice essentially distributes the torque provided by the input shaftbetween the output shafts. However, the necessity for a differentialwhich limits the differential rotation between the output shafts toprovide traction on slippery surfaces is well known.

The completely open differential, i.e. a differential without clutchesor springs, is unsuitable in slippery conditions where one wheelexperiences a much lower coefficient of friction than the other wheel,for instance, when one wheel of a vehicle is located on a patch of iceand the other wheel is on dry pavement. In such a condition, the wheelexperiencing the lower coefficient of friction loses traction and asmall amount of torque to that wheel will cause a "spin out" of thatwheel. Since the maximum amount of torque which can be developed on thewheel with traction is equal to torque on the wheel without traction,i.e. the slipping wheel, the engine is unable to develop any torque andthe wheel with traction is unable to rotate. A number of methods havebeen developed to limit wheel slippage under such conditions.

Prior methods of limiting slippage between the side gears and thedifferential casing use a frictional clutch mechanism, either clutchplates or a frustoconical engagement structure, and a bias mechanism,usually a spring, to apply an initial preload between the side gears andthe differential casing. By using a frictional clutch with an initialpreload, for example a spring, a minimum amount of torque can always beapplied to the wheel having traction, i.e. the wheel located on drypavement. The initial torque generates gear separating forces whichfurther engage the frictional clutch and develop additional torque.Examples of such limited slip differentials are disclosed in U.S. Pat.Nos. 4,612,825 (Engle), 5,226,861 (Engle) and 5,556,344 (Fox), which areassigned to the assignee of the present invention and expresslyincorporated herein by reference.

The initial preload initiates the development of side gear separatingforces which provide further braking action between the side gears andthe differential casing. In general, gear separating forces are forcesinduced on any set of meshing gears by the application of torque to thegears and tend to separate the gears. In a differential, the developmentof torque will create side gear separating forces which tend to move theside gears away from the pinion gears. When one wheel is on a surfacehaving a low coefficient of friction, the initial preload creates somecontact and frictional engagement between the differential casing andthe clutch mechanism disposed between the side gears and thedifferential casing to allow the engine to provide torque to the wheelhaving traction. This initial torque transfer induces gear separatingforces on the side gears which tend to separate the side gears tofurther frictionally engage the clutch mechanism with the casing. Theincreased frictional engagement of the clutch allows more torque to bedeveloped, thus further increasing the side gear separating forces andlimiting the slippage between the side gears and the differentialcasing.

However, such preloaded clutches are usually always engaged, and thusare susceptible to wear, causing undesirable repair and replacementcosts. Additionally, such clutch mechanisms usually employ springmechanisms which add to the cost and difficulty of manufacture.

Additionally, such a preloaded clutch mechanism may lock the outputshafts together in situations where differential rotation is necessary.For example, if the vehicle is making a turn when the wheels aresufficiently engaged on the road surface and a sufficient amount oftorque is developed, the differential will tend to lock up the outputshafts due to the action of the side gear separating forces created bythe developed torque. This may occur, for example, during turns onsurfaces with a high coefficient of friction while under acceleration.In such a case, even though differential rotation is required, thetorque and side gear separating forces lock up the two output shaftscausing one wheel to drag and slide along the road surface. This problemis evident in rear drive vehicles during turns under acceleration as theportion of the vehicle near the dragging wheel may tend to bounce up anddown.

Another method of limiting slippage involves engaging a frictionalclutch mechanism between the side gears and the differential casingbased on the difference in rotational speeds between the two outputshafts. Limited slip differentials employing this method are classifiedas speed-sensitive differentials. The frictional clutch may be actuatedby various hydraulic pump mechanisms which may be external to thedifferential casing or may be constructed of elements disposed insidethe differential casing. However, such mechanisms usually arecomplicated and also add to the cost and difficulty of manufacture.Further, speed sensitive differentials are "reactive", i.e., they reactafter a wheel has already lost traction.

A prior art method of limiting slippage involves using a flyweightgovernor in combination with a clutch mechanism wherein the governoractuates the clutch mechanism when a predetermined differential rotationrate is detected. However, prior art devices using such arrangements areconfigured such that the governor almost instantaneously appliesextremely high clutch torque to the output shafts, essentially lockingthe two output shafts together. Applying locking torque in such a mannerapplies very high stresses on the output shafts and may result infracturing the output shafts.

The above described methods actuate a clutch mechanism using mechanicalor hydraulic arrangements. It is desirable to control the actuation of alimited slip feature using electronic control methods. Electroniccontrol methods provide the advantages of accurate, reliable controlwithin a narrow control band. Electronic control methods also allowoperating parameters to be easily changed, for example by programmingthe electronic control systems to respond to a particular range ofdifferentiation speeds or some other vehicle parameter such as throttleposition.

Thus, what is needed is a simple, durable and reliable controllabledifferential which can effectively provide torque to the wheel withtraction.

What is also needed is a controllable differential which applies apredetermined amount of clutch torque in response to a predetermineddifferentiation condition of the differential.

What is also needed is a controllable differential which iselectronically actuated to provide precise, and reliable control inproviding torque to the wheel with traction.

What is also needed is a controllable differential which provides apredetermined amount of clutch torque each time a clutch mechanism iselectronically actuated.

What is also needed is a controllable differential which provides anamount of clutch torque which depends on the conditions detected byelectronic sensors, for example wheel speed sensors.

What is also needed is a controllable differential which is responsiveto speed difference to provide the limited slip or locking function onlywhen required, i.e., limited slip or lock when one wheel has losttraction, but operates as an open differential under normal tractionconditions.

Lastly, what is also needed is a controllable differential which appliesa predetermined amount of clutch torque over a predetermined period oftime in response to a loss of traction.

SUMMARY OF THE INVENTION

The present invention is a controllable differential having a clutchmechanism which transfers torque between a differential casing and aside gear disposed therein in response to the application of aninitiating force by an electronic actuator. The clutch mechanismcomprises a cone clutch element and an insert disposed between the sidegear and the rotatable casing, wherein the cone clutch element and theinsert include complementary frustoconical engagement surfaces and areadapted to move axially with respect to the rotatable casing. The coneclutch element and the side gear include camming portions having rampsurfaces which interact to produce a predetermined amount of axialmovement by the cone clutch element with respect to the side gear whenthe initiating force is applied by the electronic actuator. Theelectronic actuator comprises an electronic control system havingsensors which sense a predetermined rotational condition of the sidegear and other selected components of the controllable differential, andan electromagnet which applies the initiating force, which may bevariable, to the cone clutch element.

During non-slipping conditions, the present controllable differentialoperates as an open differential wherein the cone clutch element isdisengaged from the insert and the cone clutch element rotates with theassociated side gear. When a predetermined rotational condition of thedifferential components is sensed, such as excessive rotation of theside gear with respect to the differential casing or excessive rotationbetween the side gears and the pinion gears, the electronic controlsystem actuates the electromagnet to apply an initiating force to thecone clutch element. The initiating force produces an initial axialmovement of the cone clutch element such that the cone clutch elementfrictionally contacts the insert and momentarily slows down with respectto the side gear. The momentary slowdown causes the cam portions tointeract and provide axial separation forces which axially move the coneclutch element and the insert by a predetermined distance to therebytransfer a predetermined amount of torque from the rotatable casing tothe side gear. The present invention comprises differential embodimentshaving ball and ramp arrangements in place of the interacting camportions.

The invention comprises, in one form thereof, a controllabledifferential which transfers a predetermined amount of torque when aninitiating force is applied, comprising a rotatable casing, a piniongear rotatably supported in the rotatable casing, a side gear rotatablysupported in the casing and coupled to an output element, the side gearmeshingly engaged with the pinion gear, a clutch disposed between theside gear and the rotatable casing, the clutch comprising a cone clutchelement and an insert adapted to frictionally engage each other, thecone clutch element and the insert adapted for axial movement withrespect to the rotatable casing to thereby transfer torque between therotatable casing and the side gear, and an electronic actuator adaptedto apply the initiating force to the clutch in response to the presenceof a predetermined rotational condition of the differential componentsto frictionally engage the cone clutch element with the insert andproduce a predetermined axial movement of the cone clutch element andthe insert to thereby transfer a predetermined amount of torque betweenthe rotatable casing and the side gear.

The invention comprises, in another form thereof, a controllabledifferential which transfers torque in relation to an actuating force,comprising a rotatable casing, pinion gears rotatably supported in therotatable casing, a side gear rotatably supported in the casing andcoupled to an output element, the side gear meshingly engaged with thepinion gears, a clutch disposed between the side gear and the rotatablecasing, the clutch comprising a cone clutch element adapted tofrictionally engage a surface of the rotatable casing to therebytransfer torque between the rotatable casing and the side gear, and anelectronic actuator adapted provide the actuating force to the clutch inresponse to an external signal provided from a vehicle control system oran independent differential control system, the clutch adapted totransfer torque between the rotatable casing and the side gear in thepresence of the actuating force.

The invention comprises, in another form thereof, a controllabledifferential, comprising a rotatable casing, a pinion gear rotatablysupported therein, a first side gear also rotatably supported in thecasing and having a camming portion, the first side gear coupled with afirst output member, a second side gear also rotatably supported in thecasing and coupled with a second output member, a first clutchengageable with the casing and having a camming portion, an electricallyactuated actuator, a camming element having an axis of rotation anddisposed between and coupling the first side gear and the first clutch,the camming element having a first camming portion and a second cammingportion, the first element camming portion rotatably coupled to thefirst clutch camming portion, said second element camming portionrotatably coupled to the first side gear camming portion, and a secondclutch coupled to the second side gear and engageable with the casing.The actuation of the first clutch by the actuator engages the firstclutch with said casing. The camming element forces the second clutchinto engagement with the rotating casing.

The invention further provides a controllable differential, comprising arotatable casing, differential gearing disposed within the casing andlinking it with an output element disposed therein, a clutch alsodisposed within the casing and adapted to transfer torque between thecasing and the output element, and an electromagnet arranged to generatea generally toroidal magnetic flux path encircling the electromagnet andmagnetically forcing the clutch into engagement with the casing, whichcomprises a nonmagnetic portion adjacent the electromagnet; the fluxpath surrounds the nonmagnetic casing portion.

BRIEF DESCRIPTION OF THE DRAWINGS

The above mentioned and other features and objects of this invention,and the manner of attaining them, will become more apparent and theinvention itself will be better understood by reference to the followingdescription of the embodiments of the invention taken in conjunctionwith the accompanying drawings, wherein:

FIG. 1 is a sectional view of a controllable differential of the presentinvention;

FIG. 2 is an enlarged fragmentary sectional view of FIG. 1 showing thedifferential in the disengaged phase;

FIG. 3 is an enlarged fragmentary sectional view of FIG. 1 showing thedifferential in the transition phase;

FIG. 4 is an enlarged fragmentary sectional view of FIG. 1 showing thedifferential in the engaged phase;

FIG. 5 is a perspective view of the cone clutch element showing thecamming portion and its ramp surfaces;

FIG. 6 is a perspective view of the side gear showing the cammingportion and its ramp surfaces;

FIG. 7 is an illustration of the design factors associated with thedesign of the camming portions;

FIG. 8 is a sectional view of a second embodiment of a controllabledifferential of the present invention, showing details of the electronicinitiating mechanism;

FIG. 9 is a sectional view of a third embodiment of the controllabledifferential of the present invention, showing a variably controlledelectronic initiating mechanism;

FIG. 10 is a sectional view of a fourth embodiment of the controllabledifferential of the present invention, including an alternativeactuating method, ramping mechanism and additional frustoconicalengagement structure;

FIG. 11 is a schematic representation of the ball ramp shown in FIG. 10;

FIG. 12 is a sectional view of a fifth embodiment of the controllabledifferential of the present invention, including an alternative casingconstruction for controlling a magnetic flux circuit, ramping mechanismand air gap configuration;

FIG. 13 is a fragmentary sectional view of the embodiment of FIG. 12,showing an alternative air gap configuration;

FIG. 14 is an enlarged view of the ball and ramp arrangement of thedifferential of FIG. 12 in the disengaged state, with emphasis on thehelical slot profiles, which are not in true section;

FIG. 15 is an enlarged view of the ball and ramp arrangement of thedifferential of FIG. 12 in a partially engaged state, with emphasis onthe helical slot profiles, which are not in true section;

FIG. 16 is an enlarged view of the ball and ramp arrangement of FIG. 12in the fully engaged state, with emphasis on the helical slot profiles,which are not in true section;

FIG. 17 is a tabulation of force and torque values for an exampledifferential having characteristics which cause the cam portions to lockupon the clutch element being subjected to a momentary, 10 poundtriggering initiation force;

FIG. 18 is a graph showing how the value of the axial force on theclutch cone element, tabulated in FIG. 17, varies over time, with timeshown qualitatively;

FIG. 19 is a tabulation of force and torque values for an exampledifferential having characteristics which cause the cam portions to lockupon the clutch element being subjected to a sustained 10 poundinitiation force;

FIG. 20 is a graph showing how the value of the axial force on theclutch cone element, tabulated in FIG. 19, varies over time, with timeshown qualitatively;

FIG. 21 is a tabulation of force and torque values for an exampledifferential having characteristics which prevent the cam portions fromlocking upon the clutch element being subjected to a momentary, 10 poundtriggering initiation force;

FIG. 22 is a graph showing how the value of the axial force on the coneclutch element, tabulated in FIG. 21, varies over time, with time shownqualitatively;

FIG. 23 is a tabulation of force and torque values for an exampledifferential having characteristics which prevent the cam portions fromlocking upon the clutch element being subjected to a sustained 10 poundinitiation force; and

FIG. 24 is a graph showing how the value of the axial force on the coneclutch element, tabulated in FIG. 23, varies over time, with time shownqualitatively; and

FIG. 25 is a graph showing the multiplication factor by which theinitiation force is amplified to yield the axial force on the clutch forgiven values of CF×RF <1.

Corresponding reference characters indicate corresponding partsthroughout the several views. Although the drawings represent severalembodiments of the present invention, the drawings are not necessarilyto scale and certain features may be exaggerated in order to betterillustrate and explain the present invention. The exemplifications setout herein illustrate embodiments of the invention, in several forms,and such exemplifications are not to be construed as limiting the scopeof the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

The embodiments disclosed below are not intended to be exhaustive orlimit the invention to the precise form disclosed in the followingdetailed description. Referring to the drawings, and particularly toFIGS. 1-4, limited slip differential 10 of the present inventioncomprises differential casing 12 which is constructed by joining casingparts 12a and 12b to form a generally cylindrical structure having innercavity 13. Inner cavity 13 is constructed to hold a differential gearassembly and includes end walls formed on the interior surfaces ofcasing parts 12a, 12b. The exterior surface of casing 12 includes flange14 formed on one side thereof for connecting differential 10 to adriving ring gear (not shown) using conventionally known means, such asthreaded fasteners (not shown). Casing 12 also includes hollow receivinghubs 16, 18 on each end, the hubs defining apertures 20, 22 foraccepting output shafts 21, 23.

Disposed within inner cavity 13 are meshingly engaged pinion gears 28,30 and side gears 24, 26. Pinion gears 28, 30 are disposed at rightangles to side gears 24, 26 and are rotatably mounted on cross pin 32.Cross pin 32 is fixedly disposed in inner cavity 13, generally near themiddle of inner cavity 13. Cross pin 32 is locked in position withincasing 12 such that pinion gears 28, 30 rotate with casing 12 aroundaxis 9 defined by hubs 16, 18. Pinion gears 28, 30 can also rotatearound axis 8 of cross pin 32.

Side gears 24, 26 are axially aligned and rotatably disposed withindifferential casing 12 to rotate about horizontal axis 9. Side gears 24,26 include internal splines 25, 27 which engage corresponding splines ofoutput shafts 21, 23. The roots and teeth of side gears 24, 26 meshinglyengage the roots and teeth of pinion gears of 28, 30 such thatdifferentiation can be effected between casing 12 and output shafts 21,23. Further, cavity 31 is disposed between side gears 24, 26.

Side gear 24 further includes elongate portion 33. Elongate portion 33includes end portion 35 which contacts thrust washer 37 which in turncontacts surface 38 of casing part 12a. The outer surface of elongateportion 33 also includes groove 34 which fittingly receives snap ring39. As indicated in FIGS. 1-4, snap ring 39 is adjacent to casingsurface 46 on one side and contacts Belleville spring 49 on the otherside.

As particularly shown in FIG. 6, side gear 24 also includes cammingportion 36 having ramp surfaces 36a disposed around the axis of sidegear 24. As described further below, camming portion 36 and rampsurfaces 36a contact and interact with camming portion 56 and rampsurfaces 56a of cone clutch element 50 to force cone clutch element 50to move axially to the left as seen in FIG. 1 with respect to side gear24 when the clutch mechanism of the present limited slip differential iselectronically actuated. Side gear 24 also moves axially in the oppositedirection. However, its extent of travel is limited by pinion gears 28and 30.

Camming portion 36 interacts with a similarly arranged camming portionof the clutch mechanism which includes cone clutch element 50 and insert40. Cone clutch element 50 is disposed between side gear 24 and insert40 and is adapted to transfer frictional torque from differential casing12 to side gear 24. As shown in FIGS. 1-5, cone clutch element 50comprises an annular element having a generally T-shaped cross section.Cone clutch element 50 includes frustoconical engagement surface 55which is adapted to frictionally contact frustoconical engagementsurface 43 of insert 40. Cone clutch element 50 also includes portion 52disposed adjacent elongate portion 33 of side gear 24. Portion 52includes surface 53 which abuts Belleville spring 49. In the disengagedposition shown in FIG. 2, Belleville spring 49 urges cone clutch element50 to the right so that frustoconical engagement surface 55 is just offand out of contact with frustoconical engagement surface 43, as shown bygap 45.

As shown in FIG. 5, cone clutch element 50 also includes camming portion56 having ramp surfaces 56a disposed around the axis of cone clutchelement 50. Ramp surfaces 56a are complementary to ramp surfaces 36asuch that in the disengaged state, ramp surfaces 36a and 56a aremeshingly engaged and fully seated within each other, and remain so ascone clutch element 50 and side gear 24 rotate at the same speed.However, when cone clutch element 50 and side gear 24 rotate atdifferent speeds, the ramp surfaces 36a and 56a ride up on each other tothereby apply axial separation forces to cone clutch element 50 and sidegear 24. As described further below, this feature is used in conjunctionwith the initiating force to fully engage cone clutch element 50 andtransfer a predetermined amount of torque each time the initiating forceis applied.

Cone clutch element 50 further includes portion 51 which is disposednear casing part 12a and inner end 62 of the electromagnet 60. In thedisengaged position shown in FIG. 2 wherein cone clutch element 50 isurged to the right by Belleville spring 49, gap 63 (FIG. 2) existsbetween clutch element portion 51 and inner end 62. When an initiatingforce is applied by electromagnet 60 to cone clutch element 50, coneclutch element 50 moves to the left and reduces the size of gap 63.

Insert 40 is disposed between cone clutch element 50 and casing part 12band is rotationally fixed so that it cannot rotate, but is axiallymovable, with respect to casing 12. Insert 40 includes end portion 41which is disposed toward casing part 12a and end portion 42 which isdisposed away from casing part 12a. Belleville spring 48 is disposedbetween casing part 12a and end portion 41 of insert 40 to thereby urgeinsert 40 to the right in FIGS. 1-4. Insert 40 also includesfrustoconical engagement surface 43 which is adapted to frictionallyengage frustoconical engagement surface 55 of cone clutch element 50 totransfer frictional torque between surfaces 43 and 55.

Based on the above, it can be seen that when the limited slip feature isnot actuated, Belleville spring 49 urges cone clutch element 50 to theright such that frustoconical engagement surfaces 43 and 55 remaindisengaged and differential 10 operates as an open differential.However, when the limited slip feature is desired, electromagnet 60applies an initiating force to cone clutch element 50 causing coneclutch element 50 to move to the left as seen in FIGS. 1-4 and resultingin an interaction between frustoconical engagement surfaces 43 and 55 tocause a momentary difference in rotational speed of side gear 24 andcone clutch element 50. Camming portions 36 and 56 will thereforeinteract to cause the clutch mechanism to engage and to transfer apredetermined amount of torque from casing 12 to side gear 24.

The initiating force initially urges cone clutch element 50 to the leftin FIGS. 1-4 to frictionally engage frustoconical engagement surfaces 43and 55. The frictional engagement momentarily slows down cone clutchelement 50 with respect to side gear 24. This momentary slow down causesramp surfaces 36a and 56a to ride up on each other and thus applyfurther axial separation forces between cone clutch element 50 and sidegear 24. The separation forces cause cone clutch element 50 to movefurther axially to the left, thereby ensuring tight seating offrustoconical engagement surfaces 43 and 55 and also causing cone clutchelement 50 and insert 40 to move axially against Belleville springs 48and 49. The axial movement limit of cone clutch element 50 and insert40, as well as the maximum ride up of ramp surfaces 36a and 56a, isreached when Belleville spring 49 is fully collapsed, as shown in FIG.4. At this point cone clutch element 50 transfers a predetermined amountof torque between casing 12 and side gear 24. It can be seen that oncethe initiating force is applied, the above-described sequence occursautomatically until the clutch mechanism is fully engaged to transfer apredetermined amount of frictional torque between casing 12 and sidegear 24. As discussed further below, the differential may be designedsuch that the clutch will disengage upon release of the initiatingforce.

Electronic actuator assembly 59 is disposed adjacent to and is connectedwith casing part 12a to provide the initiating force to initially movecone clutch element 50. Electronic actuator assembly 59 comprisesannular in cross-section and generally T-shaped electromagnet 60 havingleg portion 61, which extends through opening 15 of casing part 12a, andinner portion 62 which is disposed near clutch element portion 51.Electromagnet 60 is connected to sensors 64 and an electronic controlsystem 67 via slip ring assembly 65 and connecting wires 66. Sensors 64sense a differentiation condition between side gear 24 and pinion gears28 and 30. The sensors can comprise sensors to measure the axle speedsor may include other vehicle parameters such as an onboard accelerometerand the like. When it is desired to actuate the limited slip feature,the electronic control system 67 energizes electromagnet 60 to therebymove cone clutch element 50 to the left. The electronic control systemmay comprise any conventionally known system or group of elementscapable of detecting a predetermined rotational condition of thedifferential components, such as the rate of differential rotationbetween output shafts 21 and 23, or rotation between pinion gears 28, 30and side gears 24, 26, and energizes electromagnet 60 when apredetermined rotational condition is detected.

The operation of the electronically initiated limited slip differentialof FIGS. 1-4 is now described. When the differential is initially in theresting position, wherein the components of differential 10 areinterconnected and assembled as described above, differential 10 isconnected to output shafts 21, 23, which are respectively coupled tostationary wheels (not shown) that are in contact with a ground surface,no force is applied to differential 10 by a vehicle engine (not shown)through a driving ring gear (not shown) connected via flange 14, so thatdifferential casing 10 is stationary. In such a resting condition,Belleville spring 48 urges insert 40 to the right such that end portion42 abuts the adjacent surface of casing part 12b, Belleville spring 49urges cone clutch element 50 to the right such that frustoconicalengagement surface 55 is just off frustoconical engagement surface 43,and cam portions 36 and 56 are fittingly engaged. The engine providestorque to the wheels in the conventional manner, namely through the ringgear, differential casing 12, cross shaft 32, pinion gears 28, 30, sidegears 24, 26, splines 25, 27, and output shafts 21, 23 to the wheels.

In the case where both wheels have traction and the engine is able toapply torque to both wheels, limited slip differential 10 operates as anopen differential. The side gear separating forces are directlytransmitted to casing 12 via end portion 35 of elongate portion 33through thrust washer 37 into casing surface 38. Thus, the side gearseparating forces have no effect on the operation of limited slipdifferential 10. Frustoconical engagement surfaces 43 and 55 remaindisengaged, side gears 24, 26 rotate freely with respect to casing 12and cone clutch element 50 rotates at the same rate as side gear 24.FIG. 2 illustrates limited slip differential 10 in the open differentialstate.

In situations where one wheel is slipping, i.e., one wheel is on asurface having a low coefficient of friction and the vehicle engine isunable to generate torque on the wheel with traction, differentiationbetween the two wheels occurs and it is desired to actuate the clutchmechanism to prevent a spin out. In such a case, the electronic controlsystem applies an initiating force by energizing electromagnet 60 when apredetermined rotational condition is detected to thereby move coneclutch element 50. Energizing electromagnet 60 applies an attractionforce to clutch element portion 51 thereby causing cone clutch element50 to initially move to the left in FIG. 3 and reduce the size of gap 63(FIG. 2).

The initial axial movement to the left of cone clutch element 50 to theleft causes frustoconical engagement surface 55 to frictionally engagefrustoconical engagement surface 43 of insert 40. Due to this frictionalengagement, cone clutch element 50 momentarily slows down with respectto side gear 24. The momentary difference in the rotation rate betweencone clutch element 50 and side gear 24 causes ramp surfaces 36a and 56ato ride up on each other. The ride up between ramp surfaces 36a and 56aproduces further axial separation forces which move cone clutch element50 further to the left. The further axial movement causes frustoconicalengagement surfaces 43 and 55 to fully engage and also cause cone clutchelement 50 and insert 40 to move together axially, thereby collapsingBelleville springs 48 and 49. The tight engagement and further axialmovement of cone clutch element 50 and insert 40 transfer apredetermined amount of torque between differential casing 12 and sidegear 24. This transition phase is shown in FIG. 3.

Maximum axial movement of cone clutch element 50 and insert 40 andmaximum ride up of ramp surfaces 36a and 56a is reached when Bellevillespring 49 is fully collapsed. FIG. 4 illustrates the arrangement oflimited slip differential 10 when the clutch mechanism is fully engaged.At this point, a predetermined amount of clutch torque is transferredthrough cone clutch element 50, and cone clutch element 50 again rotatesat the same speed as side gear 24. When the limited slip feature is nolonger required, i.e., both wheels have traction, electromagnet 60 isde-energized, Belleville springs force cone clutch element 50 and insert40 back to the original positions and limited slip differential returnsto the open state.

It is to be noted that once the initial axial movement of cone clutchelement 50 is induced by electronic actuator 59, the interaction betweenfrustoconical engagement surfaces 43 and 55 and between ramp surfaces36a and 56a ensure that cone clutch element 50 becomes fully engaged anda predetermined amount of torque is transferred. In other words, everytime electronic actuator 59 initiates the movement of cone clutchelement 50 by energizing electromagnet 60 to apply an attraction forceto clutch element portion 51, the elements of cone clutch element 50,side gear 24 and insert 40 are set in motion to provide thepredetermined amount of axial movement and to transfer a predeterminedamount of frictional torque between casing 12 and cone clutch element50. The design characteristics of Belleville springs 48 and 49 and thearrangement of frustoconical engagement surfaces 43 and 55 may bechanged to vary the amount of clutch torque which is transferred.

A second embodiment of the present invention is shown in FIG. 8. Onedifference between the second embodiment and the embodiment shown inFIGS. 1-4 lies in the arrangement of electronic actuator 59. As shown inFIG. 8, in controllable differential 10a, electromagnet 60a is disposedoutside of rotatable casing 12 and does not rotate. The actuatingassembly for initiating the movement of cone clutch element 50acomprises plate 70 connected to actuator element 73 by screw 72. Screw72 is disposed inside spacer 71 which is disposed inside opening 15a ofcasing part 12a. Actuator element 73 is generally L-shaped incross-section and includes leg portions 73a and 73b. Groove 75 isdisposed in leg portion 73b and is adapted to receive snap ring 74 whichabuts thrust bearing 78. Thrust bearing 78 also abuts snap ring 76 whichis disposed in groove 77 of cone clutch element 50a. Cone clutch element50a is substantially similar to cone clutch element 50 of the embodimentshown in FIGS. 1-4 except that cone clutch element 50a includes clutchelement portion 51a which is adapted to accept snap ring 76 and interactwith thrust bearing 78 via snap ring 76.

Nonrotating electromagnet 60a is supported on rotatable casing part 12aby thrust bearing 79, thus obviating the need for a slip ringarrangement. In this embodiment, the actuating assembly, comprisingplate 70, spacer 71 and actuator element 73, rotates with casing 12 andrelative to stationary electromagnet 60a.

The operation of controllable differential 10a with regard to the clutchmechanism is similar to that of the first embodiment. When theelectronic control system detects a predetermined rotational conditionindicating slippage, electromagnet 60a is energized, thereby providingan attraction force on plate 70 and urging plate 70 to move to the leftas viewed in FIG. 8. The axial movement of plate 70 to the left istransmitted to cone clutch element 50a via spacer 71, actuator element73, snap ring 74, thrust bearing 78 and snap ring 76. Again, the initialmovement of cone clutch element 50 frictionally engages frustoconicalengagement surfaces 43 and 55 thereby causing a momentary difference inthe rate of rotation between cone clutch element 50a and side gear 24.This momentary difference causes ramp surfaces 36a and 56a of camportions 36 and 56, respectively, to ride up on each other therebyproducing further axial movement of cone clutch element 50a to the left,away from side gear 24. The axial movement continues until the clutchmechanism is fully engaged, i.e., Belleville spring 49 is fullycollapsed, and transfers a predetermined amount of frictional torque,between casing 12 and side gear 24.

A third embodiment of the present invention is shown in FIG. 9. Adifference between the third embodiment and the second embodiment isthat the third embodiment does not include insert 40 nor Bellevillespring 48. Here, frustoconical engagement surface 55 of cone clutchelement 50a frictionally contacts casing part 12c. The lack of insert 40and Belleville spring 48 allows limited slip differential 10b of thethird embodiment to transfer a variable amount of frictional torquebetween casing 12 and cone clutch element 50a based on an actuatingforce provided by electromagnet 60a. The movement of cone clutch element50a is initiated in similar fashion to the second embodiment, namely, byenergizing nonrotating electromagnet 60a to move plate 70, spacer 71 andactuator element 73 to the left as viewed in FIG. 9, thereby causingcone clutch element 50a to move to the left in a manner similar to thatof the second embodiment, shown in FIG. 8. However, in the case of thethird embodiment, the movement of cone clutch element 50a does notresult in ramp surfaces 36a and 56a automatically reaching maximumride-up, Belleville spring 49 fully collapsing and cone clutch element50a moving a predetermined axial distance. Instead, the amount of axialmovement and the amount of frictional torque transferred depends on theactuating force applied by electromagnet 60a as controlled by controlsystem 67a. Based on the analysis of the ramp surface and frustoconicalengagement surface geometries described below, the third embodiment isarranged so that an actuating force does not automatically result infull engagement of the clutch mechanism. Instead the amount offrictional torque transferred depends on the amount of force applied byelectromagnet 60a, which varies with the amount of current therethrough,controlled by control system 67a. This can be accomplished either bycurrent control or by pulse width modulation. The current level isdetermined by the parameters of the control system's programming, basedon the input signals received from the vehicle sensors 64. Highercurrent creates more magnetic attraction force between electromagnet 60aand plate 70. When the magnetic force is removed, Belleville spring 49urges cone clutch element 50 to move to the right as viewed in FIG. 9such that frustoconical engagement surface 55 disengages from casingpart 12c and controllable differential 10b returns to the opendifferential state. Further, although differential 10b is used withcontrol system 67a, the scope of the present invention should not beconstrued as being so limited, and control system 67 may instead be usedto control differential 10b.

Referring now to FIGS. 5-7, the geometries of ramp surfaces 36a, 56a andcone clutch elements 50, 50a of differentials 10, 10a, 10b are some ofthe factors which determine whether the separation forces on rampsurfaces 36a, 56a will continue to increase and add force to cone clutchelement 50, 50a to fully engage the clutch mechanism or will becomereduced to zero, adding nothing to the ability of cone clutch element50, 50a to engage casing 12. In other words, these geometries determinewhether the cone clutch element 50, 50a engages in a self-progressivemanner when an initiating force is applied or whether the cone clutchelement will disengage with removal of the initiating force. As will beseen, while self-progressive, full clutch engagement results in highertorque transfer capabilities between the casing and the axles throughthe clutch, complete control over clutch disengagement is sacrificed.Conversely, to maintain complete control over clutch disengagementlimits the clutch's torque transfer capability to a maximum, albeitdefinable, limit lower than the fully engaged level. The relationshipsbetween the design elements of cone clutch element 50, 50a and rampsurfaces 36a, 56a are now described with references to FIG. 7 whichshows camming portions 36 and 56 laid out along an axis which isperpendicular to axis 9.

Factors necessary to describe the characteristics of cone clutch element50, 50a include:

μ_(c) =coefficient of friction between the frustoconical engagementsurfaces 43 and 55;

α=angle of frustoconical engagement surface 55 from axis 9; and

R_(c) =mean radius of frustoconical engagement surface 55 from axis 9.

These variables determine the "Cone Factor," CF_(c) defined as:

    CF=(R.sub.c ×μ.sub.c)/sin α

Factors necessary to describe the characteristics of ramp surfaces 36a,56a include:

μ_(r) =coefficient of friction between ramp surfaces 36a, 56a;

φ=angle of ramp surfaces 36a, 56a; and

R_(r) =mean radius of ramp surfaces 36a, 56a from axis of rotation 9.

In this case, angle φ is measured from a plane perpendicular to axis ofrotation 9 and indicates the degree of rise in the ramp surfaces. Forexample, an angle φ of zero degrees indicates flat ramp surfaces. Asangle φ increases, the degree of rise in the ramp surfaces becomessteeper. Thus, it can be envisioned that as angle φ increases, theability to transfer loads axially between surfaces 36a and 56adecreases. Also, mean radius R_(r) is a measure of the distance fromaxis of rotation 9 to the center of the ramp and is determined by theaverage of the inner and outer radii of the ramp surfaces. Thesevariables are combined to define the "Ramp Factor," RF_(c) as: ##EQU1##The "Separation Force," F_(S), attributed to the relative rotation ofslidably engaged camming portions 36a, 56a is defined as:

    F.sub.S =T.sub.r ×RF

where Tr is the torque on the camming portions.

The factors determining CF and RF may be varied according to design andperformance requirements. When CF×RF >1 or, it is expected, whenCF×RF=1, differential 10, 10a or 10b will respond differently toexternal initiating force F_(e) applied on cone clutch member 50 or 50aby electromagnet 60 or 60a than when CF×RF<1. Notably, F_(e) must begreater than the opposing force exerted on the cone clutch member byBelleville spring 49 for the clutch to engage at all. Whether theproduct of CF and RF is greater than or less than one determines whetherthe torque on ramp surface 56a will produce sufficient separation forceto self-progressively increase the "Cone Torque," T_(c), defined as:

    T.sub.c =CF×F.sub.c

where F_(c) is the axial force bearing on the cone.

When CF×RF>1 and momentary, triggering initiation force F_(e) isapplied, as when electromagnet 60 or 60a is energized and thenimmediately de-energized, cone clutch element 50 or 50a travels, over avery brief period of time, the full amount of predetermined axialmovement, frictionally contacting clutch surfaces 43 and 55, and thencontinues to self-progressively increase the clutch engagement levelautomatically to its filly engaged point. When external initiation forceF_(e) is sustained, as when the current to electromagnet 60 or 60a ismaintained over time, cone clutch element 50 or 50a travels the fullamount of its axial movement, frictionally contacting clutch surfaces 43and 55, in approximately the same short period. However, with initiatingforce F_(e) sustained, the resulting cone torque T_(c) progresses at asubstantially faster rate, vis-a-vis triggering initiating force F_(e),to the fully engaged point. In either case of a triggering or sustainedexternal force, when CF×RF>1 clutch engagement self-progresses uponaxial separation of cone clutch element 50, 50a and side gear 24, anddoes not disengage until the relative torque on ramp surfaces 36a, 56ais completely removed, which will normally occur when the axles ordifferential casing are torsionally unloaded.

Referring now to FIGS. 17-20, example clutch performance data for adifferential such as differential 10, 10a, or 10b having CF×RF>1 isprovided. FIG. 17 comprises a table illustrating the case of a 10 poundtriggering initiating force F_(e). In this example, μ_(c) is 0.1, α is 9degrees and R_(c) is 1.654 inches, resulting in CF of 1.0573; μ_(r) is0.05, φ is 32 degrees and R_(r) is 1.321 inches, resulting in RF of1.0867. Thus, CF×RF is 1.1489. Referring to FIG. 17, the followingrelationships between the tabulated variables hold at each point in timet, where time t-1 is the point in time immediately preceding time t:

    F.sub.ct= F.sub.et+ F.sub.st-1 ;

    F.sub.st= T.sub.rt× RF;

    F.sub.ct= F.sub.et +(T.sub.rt-1× RF);

    T.sub.ct= F.sub.ct× CF;

    T.sub.rt =T.sub.ct ; and

    F.sub.ct= F.sub.et +(T.sub.ct-1× RF).

It can be seen that only a momentary triggering external force F_(e) isnecessary to initiate self-progressive engagement of the clutch. FIG. 18is a graph illustrating how F_(c) of FIG. 17 progresses over time, withtime represented qualitatively rather than quantitatively.

FIG. 19 comprises a table illustrating the case of a sustained 10 poundinitiating force F_(e), all other characteristics of the differentialand its operation, and the mathematical relationships between thetabulated variables are equivalent to the previous example. FIG. 20 is agraph illustrating how F_(c) of FIG. 19 progresses over time, with timerepresented qualitatively rather than quantitatively. As can be seen,with a sustained initiating force F_(e) the clutch self-progressivelyengages at a substantially higher rate than experienced with only amomentary triggering force. As earlier mentioned, in either case of atriggering or sustained initiating force F_(e), if CF×RF>1 the clutchwill not disengage until either the casing or the axles are torsionallyunloaded; as seen in the triggering F_(e) examples of FIGS. 17 and 18,de-energizing the electromagnet providing the initiating force will notcause the clutch to disengage.

When CF×RF<1 clutch engagement does not self-progressively increase.Rather, the clutch will engage to a maximum, definable level. Further,the amount of frictional torque transferred from casing 12 to coneclutch element 50, 50a will vary with the actuating force F_(e) applied,and when F_(e) is removed, the axial separation force between coneclutch element 50, 50a and side gear 24 is overcome by the opposingaxial force applied by Belleville spring 49 and disappears, thedifferential returning to the open differential state. Thus, by settingthe variables defining CF and RF such that CF×RF<1, greater clutchcontrol can be had vis-a-vis the case where CF×RF>1, although theachievable clutch engagement force F_(c) and torque T_(c) arecomparatively much smaller. In differentials 10 or 10a, in whichconstant current levels are provided to electromagnet 60 or 60a bycontrol system 67, where CF×RF<1, the clutch may be fully engaged byenergizing the electromagnet and disengaged by merely de-energizingsame. Similarly, in differential 10b, in which variable current levelsare provided to electromagnet 10a by control system 67a, the clutch maybe increasingly or decreasingly engaged by correspondingly varying thecurrent through electromagnet 60a and disengaged by merely de-energizingsame.

When CF×RF<1 and momentary, triggering external force F_(e) is applied,as when electromagnet 60 or 60a is energized and then immediatelyde-energized, cone clutch element 50 or 50a travels, over a very briefperiod of time, the full amount of predetermined axial movement,frictionally engaging clutch surfaces 43 and 55, but the clutchmechanism does not reach its maximum engagement limit, for the onlyaxial force exerted on cone clutch element 50 or 50a is F_(e). Clutchengagement is not self-engaging as in the case where CF×RF>1. WhenCF×RF<1 and external force F_(e) is sustained, as when the current toelectromagnet 60 or 60a is maintained, cone clutch element 50 or 50aexperiences the full amount of its axial travel in the same shortperiod. However, the resulting cone torque T_(c) quickly progresses to amaximum level. In either case of triggering or sustained F_(e), whereCF×RF<1, the clutch disengages under the force of Belleville spring 49immediately upon the de-energizing of the electromagnet.

Referring now to FIGS. 21-24, example clutch performance data for adifferential such as differential 10, 10a, or 10b having CF×RF <1 isprovided. FIG. 21 comprises a table illustrating the case of a 10 poundmomentary triggering external force F_(e). In this example, theclutch-defining variables are equivalent to those of the previous case,where CF×RF>1, viz., μ_(c) is 0.1, α is 9 degrees and R_(c) is 1.654inches, resulting in CF of 1.0573. Similarly, cam ramp-definingvariables μ_(r) and R_(r) remain at 0.05 and 1.321 inches, respectively.Cam angle φ, however, is increased to 46 degrees, providing RF of0.6613. Thus, in this case CF×RF is 0.6991. As in the previous case, thefollowing relationships between the variables tabulated in FIG. 21 holdat each point in time t, where time t-1 is the point in time immediatelypreceding time t:

    F.sub.ct= F.sub.et+ F.sub.st-1 ;

    F.sub.st= T.sub.rt× RF;

    F.sub.ct= F.sub.et +(T.sub.rt-1× RF);

    T.sub.ct= F.sub.ct× CF;

    T.sub.rt =T.sub.ct ; and

    F.sub.ct= F.sub.et +(T.sub.ct-1× RF).

It can be seen that a momentary triggering external force F_(e) resultsin only a very brief period of quickly diminishing clutch engagement inwhich F_(c) reaches only the level of F_(e). FIG. 22 is a graphillustrating how F_(c) of FIG. 21 diminishes over time, with timerepresented qualitatively rather than quantitatively.

FIG. 23 comprises a table illustrating the example of a sustained 10pound external force F_(e), all other characteristics of thedifferential and its operation, and the relationships between thetabulated variables are equivalent to the previous, triggering forceexample. FIG. 24 is a graph illustrating how F_(c) of FIG. 23 progressesover time to a maximum level, with time represented qualitatively ratherthan quantitatively. As can be seen, with a sustained external forceF_(e), the clutch progressively engages from the point at which F_(e) isexperienced to plateau at a substantially higher level, hereapproximately 3.5 times F_(e). Thus, with CF×RF<1 and a sustainedexternal force F_(e), the characteristics of the clutch and its cammingarrangement can be adapted to provide a constant multiplier effectbetween external force F_(e) and the axial force on the cone, F_(c).FIG. 25 graphically illustrates the calculated constant multiplierrelating F_(e) and F_(c) for given values of CF×RF<1. As can be seen, bymanipulating the cone clutch and camming ramp factors such that theproduct of CF×RF approaches 1, F_(e) may be considerably amplified toproduce a much greater F_(c) value. With use of a variable outputcontrol system such as 67a, Fe, and thus F_(c) and T_(c) may be variablyengaged and, because the clutch engagement is not self-progressing, theengagement level may be variably or altogether diminished in response toimproved traction conditions sensed by sensors 64.

A fourth embodiment of the present invention, differential 10c, is shownin FIG. 10. The fourth embodiment includes several differences from theprevious embodiments. First, nonrotating electromagnet 60b, relative towhich rotatable casing 12 moves, directly applies an initiating force oncone clutch element 50b via a magnetic flux circuit rather than applyingthe force through a spacer 71, plate 70 and actuator element 73. Second,ball and ramp arrangement 36b, 58, 56b, disposed on the side gear 24aand cone clutch element 50b, induces additional axial movement betweenside gear 24a and cone clutch element 50b after the application of aninitiating force. Third, second cone clutch element 101, added between asecond side gear 100 and the rotatable casing 12, provides additionaltorque bias.

As shown in FIG. 10, nonrotating electromagnet 60b is disposed adjacentendcap 12e. Endcap 12e is constructed of a suitable nonmagnetic orparamagnetic material in order to create the magnetic flux circuitdescribed further below. Such nonmagnetic materials may include, but arenot limited to, AISI 300 series stainless Austenitic steel andmanganese-nickel steel, which are paramagnetic materials having relativemagnetic permeabilities very close to 1 (equivalent to air).Electromagnet 60b is also supported by bearing 79a, which maintains airgap 120 axially and radially separating the nonrotating electromagnetfrom rotating casing parts 12d, 12e and field ring 96.

Annular attractor ring 98 is attached to axial inside surface 122 ofendplate 12e using threaded fasteners 72b, thus attractor ring 98rotates with casing 12 about axis 9. A plurality of openings 124 isprovided in endplate 12e, equally distributed about axis 9. Hollowcylindrical spacers 97 extend through openings 124, one of the axialends of each spacer 97 abutting attractor ring 98. Annular field ring 96abuts annular axial outside surface 128 of endplate 12e and the otheraxial end of each spacer 97. Fasteners 72a extend through andinterconnect attractor ring 98, spacers 97, and field ring 96. Attractorring 98 and end surface 93 of threaded fastener 72a are disposedadjacent surface 94 of cone clutch element 50b. Air gap 95 existsbetween surface 94 and attractor ring 98 and threaded fastener 72a.Field ring 96, spacer 97, attractor ring 98, fastener 72a, cone clutchelement 50b and casing part 12d are made of suitable ferromagneticmaterials for conducting the magnetic flux created by electromagnet 60b,fasteners 72a and spacers 97 providing a plurality of magnetic fluxpaths through openings 124 of nonmagnetic endcap 12e.

It can be seen that the present configuration creates a magnetic fluxcircuit 80 which moves cone clutch element 50b very slightly to the leftas shown in FIG. 10 and reduces the size of air gap 95 whenelectromagnet 60b is energized. Surface 55 of cone clutch element 50bmoves only in the range of 0.001-0.002 inch toward surface 43 of casingpart 12d under the influence of the magnetic field, which corresponds toa decrease in air gap 95 of approximately 0.010 inch as primary coneclutch element 50b moves axially toward endcap 12e. The force on element50b changes with the amount of flux created by electromagnet 60b. Whenelectromagnet 60b is energized by control system 67a supplying variablecurrent levels to coil windings 106, a correspondingly variable level ofmagnetic flux is generated along path 80. The magnetic flux travels fromelectromagnet 60b across air gap 120, through casing part 12d and coneclutch element 50b, across air gap 95, through attractor ring 98,fastener 72a, spacer 97, field ring 96, and back across air gap 120 toelectromagnet 60b. Because endcap 12e is constructed of nonmagnetic orparamagnetic material, the magnetic flux in the materials of highmagnetic permeability travels around endcap 12e. Due to the magneticflux circuit, cone clutch element 50b is moved to the left as shown inFIG. 10 by the attraction forces between two pairs of surfaces, namely,between axial surfaces 93 and 94, and between frustoconical surfaces 43and 55.

The force with which cone clutch element 50b is drawn to the leftdepends on the strength of the magnetic flux field generated by coilwinding 106 of electromagnet 60b. The amount of magnetic flux created bycoil 106 is variable and is determined by the number of turns of wire inthe coil and the current flowing through the wire. This is commonlyreferred to as "NI", where N is the number of turns in the coil and I isthe current. The amount of current varies and is controlled by controlsystem 67a as described above with regard to the third embodiment.

The actuation sequence created by the momentary difference in rotationalspeed between cone clutch element 50b and side gear 24a as frustoconicalsurfaces 43 and 55 seat against each other is also similar to that ofthe third embodiment. The variable coil current induces a variableamount of magnetic clamping force between casing part 12d and coneclutch element 50b, which induces a variable amount of torque to beexerted by casing part 12d on cone clutch element 50b. Unlike the thirdembodiment (FIG. 9), however, the fourth embodiment (FIG. 10) providesthe additional axial force between cone clutch element 50b and side gear24a through use of a ball and ramp arrangement rather than by a cammingarrangement between the side gear and the cone clutch element.

Referring to FIGS. 11A-11D, ball and ramp arrangement 36b, 58, 56b iscomprised of a plurality of paired spiral slots 36b, 56b located in sidegear 24a and cone clutch element 50b, respectively, which define ahelically ramping path followed by ball 58, which may be steel, disposedin each slot pair, the ramp angle defined as angle φ_(a) (FIG. 11D). Inthe illustrated embodiment, six paired spiral slots 36b, 56b and sixballs 58 are used, although it is anticipated that as few as three slotpairs and three balls may suffice. FIGS. 11B-D illustrate the action foran individual pair of spiral slots 36b, 56b and its associated ball 58.

During normal operation with electromagnet 60b deactivated, surfaces 114and 116 of side gear 24a and cone clutch element 50b, respectively, areclosely adjacent and separated by plane 118 (FIG. 11B), which isstationary relative to the differential, parallel to surfaces 114, 116and perpendicular to axis 9. Surfaces 114 and 116 may barely contact oneanother at plane 118, but are preferably slightly separated, byapproximately 0.0001 to 0.0003 inch, by balls 58, which are seated inslots 36b, 56b. Balls 58 are urged into the deepest portion of slots36b, 56b by Belleville spring 49 and by gear separation forces betweenside gear 24a and pinion gears 28 which urge side gear 24a leftward asviewed in FIG. 10. Belleville spring 49 abuts snap ring 39 disposed inannular groove 34a in side gear 24a, and urges cone clutch element 50brightward, axially away from snap ring 39 (FIG. 10).

In operation, as electromagnet 60b is activated, axial separation ofcone clutch element 50b and side gear 24a is induced as cone clutchelement 50b is magnetically pulled to the left against the force ofBelleville spring 49 into clutched engagement with casing part 12dthrough frustoconical surfaces 43 and 55. As seen in FIG. 11C, inresponse to the initial flow of magnetic flux cone clutch element 50b ispulled to the left relative to stationary plane 118, whereas side gear24a temporarily maintains its position relative thereto. As cone clutchelement 50b and side gear 24a separate axially, ball 58 is caused torotate along the ramping helical paths of slots 56b, 36b due to therelative rotation between cone clutch element 50b and side gear 24a, asshown in FIG. 11C. As ball 58 rotates further along the helical ramppaths, it induces further axial separation of cone clutch element 50band side gear 24a. Referring now to FIGS. 10 and 11D, as frustoconicalsurface 55 of cone clutch element 50b frictionally engages frustoconicalsurface 43 of the casing, the relative rotation between clutch element50b and side gear 24a momentarily increases such that ball 58 ridesfurther along the helical paths of slots 56b, 36b. Because theengagement of surfaces 55 and 43 limit further leftward travel of coneclutch element 50b, side gear 24a is forced towards the right away fromstationary plane 118 in response to ball 58 riding further along thehelical ramp paths of slots 56b, 36b, further compressing Bellevillespring 49 and moving against the opposing pinion gear separation forces.The ball and ramp arrangement of the fourth embodiment eliminates thefriction between the cam ramp surfaces of the embodiments shown in FIGS.1-9, thus μ_(r) is effectively zero for ball and ramp arrangements.However, it is to be understood that the previously described cammingarrangement may be used with the clutch actuating arrangement containedin the fourth embodiment and conversely, ball and ramp arrangements maybe substituted for the camming arrangements.

The fourth embodiment also includes second cone clutch element 101disposed between side gear 100 and casing part 12d (FIG. 10). Coneclutch element 101 is operatively coupled with side gear 100 usingconventionally known methods, such as splining, such that cone clutchelement 101 and side gear 100 rotate together. The use of second coneclutch element 101 provides additional torque transfer capacity to thepresent embodiment. When cone clutch element 50b moves to the left asshown in FIG. 10 due to the application of an actuating force, andfurther relative axial movement is generated between cone clutch element50b and side gear 24a via the ball and ramp arrangement, axial force istransmitted by side gear 24a through transfer block 82, side gear 100and cone clutch element 101 along paths 108. Transfer block 82 isdisposed about cross pin 32 and adapted to move laterally relativethereto along axis 9. Viz., as seen in FIG. 10, as side gear 24a axiallyseparates from cone clutch element 50b , moving to the right of plane118 (FIG. 11D), side gear surface 130, which is in sliding engagementwith or only slightly separated from surface 132 of transfer block 82,forces transfer block 82 to the right. Surface 134 of the rightwardlymoving transfer block, which is in sliding engagement with or onlyslightly separated from surface 136 of side gear 100, forces gear 100rightward. Annular surface 138 of side gear 100 abuts surface 140 ofcone clutch element 101, which rotates with side gear 100. Cone clutchelement 101 is thus driven rightwardly, its surface 105 enteringfrictional engagement with surface 104 of casing part 12d.

Thus, the force along paths 108 urges cone clutch element 101 to theright as shown in FIG. 10, thereby engaging frustoconical surfaces 105and 104 to transfer additional frictional torque from casing 12 to axles21a, 23a splined to side gears 24a, 100, respectively. Therefore, theforce along paths 108 in combination with cone clutch element 101 andthe casing part 12d increases the torque transfer capacity ofdifferential 10c. Further, the separating forces between pinion gears 28and side gear 100 are not directly grounded to casing part 12d, insteadbeing applied to cone clutch element 101. Thus, differential 10c istorque sensitive.

FIG. 10 also shows two alternative flange positions 112 and 113 oncasing part 12d which may be used to accommodate different ring gearoffsets. It is to be understood that either of these two flangepositions may be used with any of the described embodiments and that thelocation of the flange positions as shown are merely illustrative andshould not be interpreted as limiting the scope of the presentinvention.

As in the case of first three embodiments, which use cammingarrangements, the geometries of ramp surfaces 36b, 56b and cone clutchelement 50b are some of the factors which determine whether theseparation forces on ball and ramp arrangement of the fourth embodimentwill continue to increase and add force to cone clutch element 50b tofully engage the clutch mechanism or will become reduced to zero, addingnothing to the ability of cone clutch element 50b to engage casing 12.In other words, these geometries determine whether the cone clutchelement 50b automatically reaches full engagement when an initiatingforce is applied or whether the degree of engagement of cone clutchelement 50b varies depending on the amount of the actuating force. Forthe clutch mechanism of differential 10c to fully engage each time anactuating force is applied, the separation forces must continuallyincrease so that cone clutch element 50b experiences the full range ofdesired movement.

The necessary relationships between the design elements of cone clutchelement 50b and ramp surfaces 36b, 56b are now described with referencesto FIGS. 11B-D, which show helical slots 36b and 56b relative tostationary plane 118 which is perpendicular to axis 9. As in the case ofdifferentials 10, 10a and 10b, the characteristics of cone clutch member50b and ball and ramp arrangement 36b, 58, 56b in differential 10c maybe chosen to yield a suitable product value for CF×RF. Factors necessaryto describe cone factor CF₁ of cone clutch element 50b include:

μ_(c1) =coefficient of friction between the frustoconical engagementsurfaces 43 and 55;

α₁ =angle of frustoconical engagement surface 55 from axis 9; and

R_(c1) =mean radius of frustoconical engagement surface 55 from axis 9.

These variables determine cone factor CF₁ of cone clutch element 50b,defined as:

    CF.sub.1 =(R.sub.c1 ×μ.sub.c1)/sin α.sub.1.

Factors necessary to describe the characteristic of helical rampsurfaces 36b, 56b include:

μ_(r) =coefficient of friction between ramp surfaces 36b, 56b(essentially zero);

φ_(a) =angle of ramp surfaces 36b, 56b; and

R_(r) =mean radius of ramp surfaces 36b, 56b.

As was angle φ in previously described embodiments, angle φ_(a) ismeasured from a plane perpendicular to axis of rotation 9 and indicatesthe degree of rise in the ramp surfaces. These variables are combined todefine the "Ramp Factor," RF, defined as: ##EQU2## Separation forceF_(s), attributed to the relative rotation of slots 36b and 56b aboutball 58 (FIGS. 11B-D), is defined as:

    F.sub.s =T.sub.r ×RF

where Tr is the magnitude of the torque on slots 36b and 56b.

The torque on ramp surface 56b equals the cone torque on primary coneclutch element 50b. Thus, T_(r) =T_(c1). Finally, the axial force oncone clutch element 50b equals the separation force on ramp surface 56b.This force is equivalent in magnitude to the force on ramp surface 36b,which is transferred through side gear 24a to transfer block 82, to sidegear 100 and to secondary cone clutch member 101, the axial force onwhich is designated F_(c2). Thus, F_(s) =F_(c2).

Factors necessary to describe the characteristics of secondary coneclutch element 101 include:

μ_(c2) =coefficient of friction between the frustoconical engagementsurfaces 104 and 105;

α₂ =angle of frustoconical engagement surface 105 from axis 9; and

R₂ =mean radius of frustoconical engagement surface 105 from axis 9.

These variables determine cone factor CF₂ of cone clutch element 101,defined as:

    CF.sub.2 =(R.sub.c2 ×μ.sub.c2)/sin α.sub.2.

It is envisioned that CF1 and CF2 would normally be equivalent, althoughthey may differ. The "Secondary Cone Torque," T_(c2), is defined as:

    T.sub.c2 =CF.sub.2 ×F.sub.c2

where F_(c2) is the axial force on cone clutch element 101, equivalentin magnitude to F_(c1). The clutch engagement and disengagementcharacteristics associated with CF×RF>1 and CF×RF<1 for triggering andsustained external forces F_(e) discussed above with regard to the thirdembodiment, differential 10b, also apply to the fourth embodiment,differential 10c. Further, although differential 10c is used withcontrol system 67a, the scope of the present invention should not beconstrued as being so limited, and control system control system 67 mayinstead be used to control differential 10c.

FIG. 12 shows differential 10d, a fifth embodiment of the presentinvention. The fifth embodiment includes two primary differences fromthe fourth embodiment, differential 10c (FIG. 10). First, differential10d comprises first ball and ramp arrangement 36c, 58a, 56c which isassociated with the engagement of primary cone clutch element 50c withinterior surface 43a of casing part 12f, and a second ball and ramparrangement 152, 154, 156 which is associated with the engagement ofsecondary cone clutch element 101 with interior surface 104a of casingpart 12f. Second, differential 10d comprises endcap 12g which includeshub portion 158, which is made of a suitable ferromagnetic material, andannular outer portion 160, which is made of a nonmagnetic material.Endcap portions 158 and 160 may comprise separate powdered metalelements made, respectively, of magnetic and nonmagnetic materials, theelements interfitted together by abutting mating frustoconical surfaces162, 164 prior to sintering, which attaches the endcap portionstogether.

Alternatively, the unsintered powdered metal elements of magnetic endcapportion 158 may be employed to form part of the mold for nonmagneticendcap portion 160. Thus, magnetic portion 158 may define, in part, theconfiguration of nonmagnetic portion 160, that part being the surface ofnonmagnetic portion 160 which interfaces magnetic portion 158. In asimilar fashion, the unsintered powdered metal element of nonmagneticendcap portion 160 may be employed to form part of the mold for magneticendcap portion 158, thus defining the surface thereof which interfacesportion 160. In either of these alternatives, the two portions 158, 160are sintered together to form endcap 12g, and the interfacing surfacesthereof need not be frustoconical.

Further, other alternative embodiments of endcap 12g may comprisemachined bar stock portions 158, 160 or separately formed and sinteredpowdered metal portions 158, 160 which are threadedly attached together,or may comprise machined bar stock portions 158, 160 which are frictionor pressure welded together.

In the embodiment shown in FIG. 12, endcap 12g is a powdered metalassembly having central hub portion 158, through which output shaft 21aextends, and annular outer portion 160 having threaded outer periphery166 which engages mating threaded surface 168 of casing part 12f. Endcaphub portion 158 is constructed of a suitable ferromagnetic material andannular outer portion 160 is constructed of a suitable nonmagnetic orparamagnetic material in order to create the magnetic flux circuitdescribed further below. Endcap portions 158, 160 are joined alongmating frustoconical surfaces 162, 164 prior to the endcap beingsintered. Central hub portion 158 engages annular outer portion 160 fromthe right as viewed in FIG. 12, the leftmost inner diameter of outerportion frustoconical surface 164 being smaller than the rightmost innerdiameter thereof. Alternatively, as shown in FIG. 13, central hubportion 158a engages annular outer portion 160a from the left, theleftmost inner diameter of outer portion frustoconical surface 164abeing larger than the rightmost inner diameter thereof. Either of thesearrangements provide, after sintering, a structurally strong endcap.Further, the inside axial surface of either portion may be milled flushto the other portion's inside axial surface, as necessary, to helpensure gap 95a is consistently sized.

As shown in FIG. 12, nonrotating electromagnet 60c is disposed adjacentendcap 12g, supported thereon by bearing 79b. Air gap 120a axially andradially separates the nonrotating electromagnet from rotating casingparts 12f and 12g. Notably, endcap 12g and electromagnet 60c comprisemating sawtooth profiled portions 172, 174, respectively, between whicha portion of air gap 120a exists. Sawtooth profiled portion 172 providesa plurality of substantially parallel, cylindrical surfaces radiallyseparated but substantially aligned axially, and a frustoconical surfaceextending between the axially opposite ends of each adjacent pair ofcylindrical surfaces. Similarly, sawtooth profiled portion 174 providesmating cylindrical and frustoconical surfaces parallel to thecorresponding surfaces of sawtooth profiled portion 172. Thisarrangement allows substantial inner axial face tolerances or relativeaxial movement between the inner and outer races of bearing 79b, for thedistance between the corresponding frustoconical surfaces varysubstantially while air gap 120a is properly maintained between matingcylindrical surfaces in the sawtooth profile. Thus, bearing 79b may beof lesser precision than otherwise necessary without greatlycompromising clutch performance. Alternatively, as shown in FIG. 13, theinterfacing surfaces of electromagnet 60d and endcap 12h may have rathersmooth, parallel contours, defining air gap 120b.

Referring again to FIG. 12, it can be seen that magnetic circuit 80a iscreated which moves primary cone clutch element 50c very slightly to theleft and reduces the size of air gap 95a when electromagnet 60c isenergized. Surface 55a of cone clutch element 50c moves only in therange of 0.001-0.002 inch toward surface 43a of casing part 12f underthe influence of the magnetic field, which corresponds to a decrease inair gap 95a of approximately 0.010 inch as primary cone clutch element50c moves axially toward endcap 12g . The force on element 50c changeswith the amount of flux created by electromagnet 60c. When electromagnet60c is energized by control system 67a supplying variable current levelsto coil windings 106, a correspondingly variable level of magnetic fluxis generated along path 80a. The magnetic flux travels fromelectromagnet 60c across air gap 120a, through casing part 12f andprimary cone clutch element 50c, across air gap 95a, through ferrouscentral hub portion 158 of endcap 12g, and back across air gap 120a toelectromagnet 60c by way, primarily, of the parallel cylindricalsurfaces of mating sawtooth profiled portions 172, 174. Because annularouter portion 160 of endcap 12g is constructed on nonmagnetic orparamagnetic material, the magnetic flux in the materials of highmagnetic permeability travels around portion 160. Due to the magneticflux circuit, cone clutch element 50c is moved to the left as shown inFIG. 12 by the attraction forces between two pairs of surfaces, namely,between the interfacing axial surfaces of ferrous endcap central hubportion 158 and element 50c, between which air gap 95a is located, andfrustoconical clutch surfaces 43a and 55a. As explained above withrespect to the fourth embodiment, the degree to which primary coneclutch element 50c of the fifth embodiment is drawn to the left dependson the strength of the magnetic flux field generated by coil winding106a.

Disposed between primary cone clutch element 50c and side gear 24b isannular cam plate 150, which has axially extending portion 151 whichabuts thrust washer 186 adjacent endcap 12g. The actuation sequencecreated by the momentary difference in rotational speed between primarycone clutch element 50c and cam plate 150 as frustoconical surfaces 43aand 55a seat against each other is similar to that of the fourthembodiment. The variable coil current induces a variable amount ofmagnetic clamping force between casing part 12f and primary cone clutchelement 50c, which induces a variable amount of torque to be exerted bycasing part 12f on element 50c. Ball and ramp arrangement 36c, 58a, 56cis comprised of a first plurality of paired spiral slots 36c, 56clocated in interfacing surfaces 176, 178 (FIGS. 14-16) of cam plate 150and primary cone clutch element 50c, respectively, in a manner similarto that described with respect to the fourth embodiment, differential10c. Slots 36c, 56c define a helically ramping path followed by ball58a, which may be steel, disposed in each slot pair, the ramp angledefined as angle φ_(b) (FIG. 15), which corresponds to ramp angle φ_(a)of the fourth embodiment (FIG. 11D).

Referring to FIG. 14 in particular, with electromagnet 60c de-energized,surfaces 176, 178 are closely adjacent and on opposite sides of 118a,which is stationary relative to the differential, perpendicular to axis9 and parallel to surfaces 176, 178. Surfaces 176, 178 may barelycontact one another, but are preferably separated by approximately0.0001 to 0.0003 inch by balls 58a, which are seated in the deepestportion of slots 36c, 56c by Belleville spring 49a, which acts onsurface 180 of groove 182 in side gear 24b and on snap ring 39a,disposed in large diameter base 183 of frustoconic recess 184 insidecone clutch element 50c. Surface 176 of cone clutch element 50c ismaintained in position relative to plane 118a when differential 10d isin its unclutched state by Belleville spring 49a urging snap ring 39a,and thus cone clutch element 50c, toward cam plate 150, each ball 58aassuming a centered position between its respective pair of rampingsurfaces 36c, 56c. Surface 178 of cam plate 150 is maintained inposition relative to plane 118a when differential 10d is in itsunclutched state by gear separation forces between pinion gears 28 andside gear 24b, which are transferred to cam plate 150 through balls 154,disposed in slots 152, 156 provided in interfacing surfaces 188, 190,urging cam plate 150 against annular thrust washer 186 (FIG. 12)disposed adjacent endcap 12g.

Interfacing surfaces 188, 190 of side gear 24b and cam plate 150,respectively, are closely adjacent and on opposite sides of plane 192when differential 10d is in its unclutched state. Surfaces 188, 190 maybarely contact each other in this state, but are preferably separated byapproximately 0.0001 to 0.0003 inch by balls 154, which are seated inslots 154, 156. In the unclutched state, balls 154 are urged into thedeepest portion of slots 154, 156 by Belleville spring 49a. Plane 192 isparallel to plane 118a and moves along axis 9 with the centers of balls154. Second ball and ramp arrangement 152, 154, 156 is comprised of asecond plurality of paired spiral slots 152, 156 located in interfacingsurfaces 188, 190 in a manner similar to that described with respect tofirst ball and ramp arrangement 36c, 58a, 56c. Each pair of slots 152,156 defines a helically ramping path followed by ball 154, which may besteel, disposed in the slot pair. The ramp angle defined as angle θ(FIG. 16) is substantially less than angle φ_(b). That angle θ is"shallower" than angle φ_(b) means that ball 154 is able to transfergreater axially directed loads than ball 58a. Thus, ball 154 issubstantially larger in diameter than ball 58a, providing a greatercontact area with slots 152, 156 than ball 58a has with slots 36c, 56c,maintaining stresses associated with the higher loads at acceptablelevels. As will be further described below, ball and ramp arrangement152, 154, 156 transfers axial forces between cam plate 150, which abutsthrust washer 186 at endcap 12g , and side gear 24, which communicateswith transfer block 82, side gear 100, secondary cone clutch element 101and surface 104a of casing part 12_(f).

In operation, as electromagnet 60c is activated, axial separation ofprimary cone clutch element 50c and cam plate 150 is induced as coneclutch element 50c is magnetically pulled to the left, as viewed in FIG.12, against the force of Belleville spring 49a into clutched engagementwith casing part 12f through frustoconical surfaces 43a and 55a. As seenin FIG. 15, in response to the initial flow of magnetic flux, coneclutch element 50c is pulled to the left relative to stationary plane118a and surfaces 43a and 55a abut, entering frictional engagement. Asprimary cone clutch element 50c and cam plate 150 separate axially, ball58a is caused to rotate along the ramping helical paths of slots 56c,36c due to the relative rotation between element 50c and cam plate 150,as shown in FIG. 15. Cam plate 150, urged against thrust washer 186 bythe force of Belleville spring 49a and gear separation forces betweenpinion gears 28 and side gear 24b, maintains its axial position relativeto plane stationary 118a. As ball 58a rotates further along the helicalramp paths, frustoconical surfaces 43a, 55a are forced into tighterfrictional engagement and cam plate 150, still abutting thrust washer186, reaches the end of its rotational travel relative to cone clutchmember 50c.

Once cam plate 150 reaches its end of travel relative to primary coneclutch member 50c, side gear 24b begins to rotate relative to cam plate150. Referring to FIG. 16, relative rotation of side gear 24b and camplate 150 causes ball 154 to rotate along the ramping helical paths ofslots 152, 156, as shown by the increased displacement of plane 192,which axially follows the centerline of ball 154, from surfaces 188,190. As viewed in FIG. 12, side gear 24b moves towards the right,forcing secondary cone clutch element 101 into abutment with casing part12f via transfer block 82 and side gear 100 in the manner describedabove with respect to the fourth embodiment shown in FIG. 10. Assurfaces 104a, 105 engage, side gear 24b reaches its end of travel,rotationally and axially, relative to cam plate 150. As ball 154 becomesmore tightly compressed between slots 152, 154, force is transferredalong lines 108a between endcap 12g, thrust washer 186, cam plate 150,ball 154, side gear 24b, transfer block 82, side gear 100, cone clutchmember 101 and casing part 12f. Because angle θ of slots 152, 156 (FIG.16) is smaller than angle φ_(b) of slots 36c, 56c (FIG. 15), a greaterengagement force is exerted on secondary cone clutch element 101 than onprimary cone clutch element 50c. It is estimated that 80 percent of thetotal torque transfer between casing 12 and axles 21a, 23a is providedby the engagement of secondary clutch surfaces 104a, 105, and only 20percent by the engagement of primary clutch surfaces 43a, 55a. The balland ramp camming arrangement of the fifth embodiment eliminates thefriction between the sliding cam ramp surfaces in the cammingarrangements of the embodiments shown in FIGS. 1-9. However, it is to beunderstood that the camming arrangement of the fifth embodiment maycomprise slidably engaging ramp surfaces in place of the first and/orsecond ball and ramp arrangements.

As in the case of the fourth embodiment, differential 10c, whichcomprises a single ball and ramp arrangement, the geometries andfrictional characteristics of first ball and ramp arrangement 36c, 58a,56c and second ball and ramp arrangement 152, 154, 156 of differential10d determine whether the separation forces on the ball and ramparrangements will continue to increase and add force to primary andsecondary cone clutch elements 50c and 101, respectively, or becomereduced to zero upon the de-energizing of the electromagnet. In otherwords, these geometries determine whether the cone clutch elements 50cand 101 automatically reach full engagement when external force F_(e) isapplied or whether the degree of engagement of the cone clutch elementsvaries depending on the amount of the external force.

The necessary relationships between the design elements of primary coneclutch element 50c and first ball and ramp arrangement 36c, 58a, 56c arenow described with references to FIG. 14, which shows ball and ramparrangement 36c, 58a, 56c laid out on stationary plane 118a, which isperpendicular to axis 9. As in the embodiments discussed above, thecharacteristics of primary cone clutch member 50c and first ball andramp arrangement 36c, 58a, 56c, and secondary cone clutch member 101 andsecond ball and ramp arrangement 152, 154, 156 in differential 10d maybe chosen to yield a suitable product values for CF×RF for each ball andramp arrangement. Factors necessary to describe cone factor CF₁ ofprimary cone clutch element 50c include:

μ_(c1) =coefficient of friction between the frustoconical engagementsurfaces 43a and 55a;

α₁ =angle of frustoconical engagement surface 55a from axis 9; and

R_(c1) =mean radius of frustoconical engagement surface 55a from axis 9.

Cone factor CF₁ of primary cone clutch element 50c is defined as:

    CF.sub.1 =(R.sub.c1 ×μ.sub.c1)/sin α.sub.1.

Factors necessary to describe the characteristics of helical rampsurfaces 36c, 56c of the first ball and ramp arrangement include:

μ_(r1) =coefficient of friction between ball 58a and ramp surfaces 36c,56c (essentially zero);

φ_(b) =angle of ramp surfaces 36c, 56c; and

R_(r1) =mean radius of ramp surfaces 36c, 56c from axis of rotation 9.

As are angles φ and φ_(a) a in previously described embodiments, angleφ_(b) is measured from a plane perpendicular to axis of rotation 9 andindicates the degree of rise in the ramp surfaces. These variables arecombined to define primary ramp factor RF₁ : ##EQU3## Primary separationforce F_(s1) associated with the relative rotation of slots 36c and 56cabout ball 58a (FIGS. 14-16), is defined as:

    F.sub.s1 =T.sub.r1 ×RF.sub.1

where T_(r1) is the magnitude of the torque on slots 36c and 56c andthus on cam plate 150 and primary cone clutch element 50c, respectively.

Factors necessary to describe the characteristics of helical rampsurfaces 152, 156 of the second ball and ramp arrangement include:

μ_(r2) =coefficient of friction between ball 154 and ramp surfaces 152,156 (essentially zero);

θ=angle of ramp surfaces 152, 156; and

R_(r2) =mean radius of ramp surfaces 152, 156 from axis of rotation 9.

As is angle φ_(b), angle θ is measured from a plane perpendicular toaxis of rotation 9 and indicates the degree of rise in the rampsurfaces. These variables are combined to define secondary ramp factorRF₂ : ##EQU4## Secondary separation force F_(s2), associated with therelative rotation of slots 152 and 156 about ball 154 (FIGS. 14-16), isdefined as:

    F.sub.s2 =T.sub.r2 ×RF.sub.2

where T_(r2) is the magnitude of the torque on slots 152, 156 and thuson cam plate 150 and side gear 24b. Because secondary ramp factor RF₂ isgreater than primary ramp factor RF₁, secondary separation force F_(s2)is greater than primary separation force F_(s1), thus ensuring cam plateportion 151 is maintained in abutting relationship to thrust washer 186,and is transferred through side gear 24b to transfer block 82, to sidegear 100 and to secondary cone clutch member 101, the axial force onwhich is designated F_(c2). Thus, F_(s2) =F_(c2). Factors necessary todescribe cone factor CF₂ of secondary cone clutch element 101 include:

μ_(c2) =coefficient of friction between the frustoconical engagementsurfaces 104 and 105;

α₂ =angle of frustoconical engagement surface 105 from axis 9; and

R_(c2) =mean radius of frustoconical engagement surface 105 from axis 9.

Cone factor CF₂ of secondary cone clutch element 101 is defined as:

    CF.sub.2 =(R.sub.c2 ×μ.sub.c2)/sin α.sub.2.

It is envisioned that CF₁ and CF₂ would normally be equivalent, althoughthey may differ. Secondary cone torque T_(c2) is defined as:

    T.sub.c2 =CF.sub.2 ×F.sub.c2

where F_(c2) is equivalent to F_(s2), as indicated above.

Moreover, upon clutch actuation, the force of cam plate 150 againstthrust washer 186 imparts a torque, T_(tw), on the cam plate defined asfollows:

    T.sub.tw =F.sub.tw ×TWF,

where F_(tw) is the axial force on thrust washer 186 and TWF is thethrust washer factor, defined as:

    TWF=μ.sub.tw ×R.sub.tw,

where

μ_(tw) =the lowest coefficient of friction between the thrust washer andan abutting surface; and

R_(tw) =mean radius of the thrust washer.

In the differential's unclutched state, F_(tw) equals the gearseparation force and T_(tw) serves to increase the time delay betweenthe point where clutch surfaces 43a, 55a engage and ball 58a reaches itsend of travel in slots 36c, 56c, thus providing smoother overall clutchengagement. If a needle roller washer is used for thrust washer 186,μ_(tw), and thus T_(tw), are essentially zero, allowing the differentialto more quickly return to its normal, open state when the electromagnetis de-energized.

For each of first and second ball and ramp arrangements in the fifthembodiment, differential 10d, the above-discussed clutch engagement anddisengagement characteristics associated with CF×RF>1 and CF×RF <1 fortriggering and sustained external forces F_(e) also apply.

While this invention has been described as having an exemplary design,the present invention may be further modified within the spirit andscope of this disclosure. This application is therefore intended tocover any variations, uses, or adaptations of the invention using itsgeneral principals. Further, this application is intended to cover suchdepartures from the present disclosure as come within known or customarypractice in the art to which this invention pertains.

What is claimed is:
 1. A controllable differential, comprising:arotatable casing, differential gearing disposed within said rotatablecasing, said differential gearing linking said casing with an outputelement disposed in said rotatable casing; a clutch disposed within saidrotatable casing, said clutch adapted to transfer torque between saidrotatable casing and said output element; and an electromagnet arrangedto generate a generally toroidal magnetic flux path encircling saidelectromagnet and magnetically forcing said clutch into engagement withsaid rotatable casing; said casing including an endcap, said endcapcomprising a nonmagnetic portion adjacent said electromagnet, and amagnetic portion, said magnetic and nonmagnetic portions secured to eachother, said flux path surrounding said nonmagnetic casing portion. 2.The differential of claim 1, wherein said magnetic and said nonmagneticendcap portions each comprise mating powdered metal elements which areinterfitted and sintered together.
 3. The differential of claim 2,wherein said magnetic and said nonmagnetic powdered metal elements areinterfitted along mating frustoconical surfaces.
 4. The differential ofclaim 1, wherein said magnetic endcap portion partly defines theconfiguration of said nonmagnetic endcap portion, said nonmagneticportion made of powdered metal.
 5. The differential of claim 1, whereinsaid nonmagnetic endcap portion partly defines the configuration of saidmagnetic endcap portion, said magnetic portion made of powdered metal.6. The differential of claim 1, wherein said magnetic and nonmagneticendcap portions are threadedly secured to each other.
 7. Thedifferential of claim 1, wherein said magnetic and nonmagnetic endcapportions are welded together.